Field of the Invention
The invention relates to a semiconductor configuration with ohmic contact-connection, as well as to a method for contact-connecting a semiconductor configuration.
The invention relates, in particular, to a semiconductor configuration of the above-mentioned type which includes a predetermined polytype of silicon carbide at least in specific semiconductor regions, in particular semiconductor regions that are contact-connected.
Silicon carbide (SiC) in monocrystalline form is a semiconductor material having outstanding physical properties which make that semiconductor material appear to be of interest particularly for power electronics. That is the case even for applications in the kV range, inter alia due to its high breakdown field strength and its good thermal conductivity. Since the commercial availability of monocrystalline substrate wafers, especially ones made of 6H and 4H silicon carbide polytypes, has risen, silicon carbide-based power semiconductor components, such as e.g. Schottky diodes, are now also receiving more and more attention. Other silicon carbide components which are becoming increasingly widespread are pn diodes and transistors such as, for example, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
Stable ohmic contacts to semiconductor regions of different conduction types are indispensable for the functioning of those components. In that case, the lowest possible contact resistances are sought in order to minimize undesirable losses at the semiconductor-metal junction.
An overview paper entitled xe2x80x9cOhmic contacts to SiCxe2x80x9d by G. L. Harris et al. from xe2x80x9cProperties of Silicon Carbide,xe2x80x9d ed. by G. L. Harris, INSPEC, 1995, pages 231-234 contains a summary of contact-connection methods for silicon carbide having different polytypes and conduction types. With regard to the contact-connection of n-conducting and p-conducting SiC, the overview paper and the cross-references cited reveal the current state of the art that is generally accepted by experts, as outlined below:
The above-mentioned overview paper only specifies methods in which silicon carbide having only a single conduction type in each case is provided with an ohmic contact.
The contact-connection of n-conducting SiC is accordingly effected through the use of a thin contact layer of a metal or through the use of a layer sequence of different materials. The contact layers are heat-treated at temperatures of between 600xc2x0 C. and 1100xc2x0 C. In particular transition metals such as nickel, for example, yield a very good ohmic contact after the thermal treatment on n-conducting, highly doped SiC. That is because at temperatures of around 1000xc2x0 C., a metal silicide is formed from the transition metal and the silicon contained in the SiC. In comparison therewith, contact techniques for n-conducting SiC which work without a corresponding thermal treatment yield a relatively high contact resistance or a current-voltage characteristic that does not correspond to Ohm""s law. Furthermore, the thermal treatment also has a positive effect on the thermal stability of the ohmic contacts being formed.
Aluminum is predominantly used for contact-connecting p-conducting SiC. Since aluminum is readily soluble in SiC and acts as an acceptor, a zone that is highly doped with aluminum can be produced in a boundary region between the aluminum-containing contact region and the semiconductor region made of SiC. In order to avoid evaporation of the aluminum, which melts at a temperature as low as 659xc2x0 C., during a subsequent thermal treatment, at least one covering layer made of a material having a higher melting point, such as e.g. nickel, tungsten, titanium or tantalum, is applied on the aluminum.
A paper entitled xe2x80x9cThermally stable low ohmic contacts to p-type 6H-SiC using cobalt silicidesxe2x80x9d by N. Lundberg, M. xc3x96stling from Solid-State Electronics, Vol. 39, No. 11, pages 1559-1565, 1996 discloses a method for contact-connecting p-conducting SiC which uses the formation of cobalt silicide (CoSi2). A very low contact resistance can be achieved with the method described and the contact material used.
A paper entitled xe2x80x9cReduction of Ohmic Contact Resistance on n-Type 6H-SiC by Heavy Dopingxe2x80x9d by T. Uemoto, Japanese Journal of Applied Physics, Vol. 34, 1995, pages L7 to L9 discloses a layer structure being formed of a titanium layer having a thickness of 15 nm and an aluminum layer having a thickness of 150 nm as a possible ohmic contact both on p-conducting and on n-conducting silicon carbide. However, a good contact resistance on the n-conducting semiconductor region is attained only when the dopant concentration in the n-conducting semiconductor region is chosen to be very high. The disclosed dopant concentration of 4.5xc2x71020 cmxe2x88x923 is considerably above the dopant concentrations that are usually used in a silicon carbide semiconductor configuration at the present time. Such a high dopant concentration can only be produced with considerable additional outlay. Thus, during ion implantation, for example, there is the risk of the n-conducting semiconductor region being damaged.
It is accordingly an object of the invention to provide a semiconductor configuration with ohmic contact-connection and a method for contact-connecting a semiconductor configuration, which overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provide improved contact-connection of n-conducting and p-conducting SiC in comparison with the prior art. In this case, the contacts on the n-conducting and p-conducting semiconductor regions in each case are intended both to have a low contact resistance and to be thermally stable. Moreover, only dopant concentrations which can be produced in a simple manner with currently available technologies are intended to be provided for the n-conducting and p-conducting SiC.
With the foregoing and other objects in view there is provided, in accordance with the invention, a semiconductor configuration with ohmic contact-connection, comprising at least one first semiconductor region made of n-conducting silicon carbide, and at least one second semiconductor region made of p-conducting silicon carbide, the n-conducting and the p-conducting silicon carbide each having a dopant concentration of between 1017 cmxe2x88x923 and 1020 cmxe2x88x923; at least one first contact region adjoining the first semiconductor region, and at least one second contact region adjoining the second semiconductor region; the first and second contact regions having an at least approximately identical material composition being practically homogeneous within the respective contact region; and the first and second contact regions formed of a material composed at least of a first and a second material component, the first material component being nickel and the second material component being aluminum.
With the objects of the invention in view, there is also provided a method for contact-connecting a semiconductor configuration, which comprises forming at least one first practically homogeneous contact region on a first semiconductor region made of n-conducting silicon carbide, and forming at least one second practically homogeneous contact region on a second semiconductor region made of p-conducting silicon carbide; applying an at least approximately identical material having a practically homogeneous material composition within each respective contact region for the first and second contact regions; providing each of the first and second semiconductor regions with a dopant concentration of between 1017 cmxe2x88x923 and 1020 cmxe2x88x923; and forming the material at least of a first and a second material component, with nickel as the first material component and aluminum as the second material component.
In this case, the invention is based on the insight that, contrary to the customary procedure employed by experts, in which ohmic contact is made with n-conducting and p-conducting silicon carbide having different material in each case, contact-connection of silicon carbide of both conduction types is nevertheless possible with a single material. This results in significant advantages during production since the process steps required for a contact material which differs therefrom are obviated.
It is advantageous for the formation of a good ohmic contact, if the first and second semiconductor regions in each case have a sufficiently high dopant concentration at least at the surface of the semiconductor region. In this case, the dopant concentrations preferably lie between 1017 cmxe2x88x923 and 1020 cmxe2x88x923. A particularly good contact results if the dopant concentration is at least 1019 cmxe2x88x923. These specifications apply both to the n-conduction and to the p-conduction type. These dopant concentrations can be produced without difficulty through the use of ion implantation, for example. In particular, they are also distinctly below the dopant concentration mentioned in the prior art.
What is crucial for the formation of a good ohmic contact resistance both on the n-conducting and on the p-conducting silicon carbide in this case is that the material is not applied to the respective semiconductor regions in the form of a layer structure but rather with a practically homogeneous material composition. If the material is composed of a plurality of material components, such homogeneous material application has the effect that, at the interface with the two semiconductor regions, in each case all of the material components are present directly and can interact with the silicon carbide of the two semiconductor regions. In contrast, in the case of a layer structure, it is necessary first of all to mix together the individual material components which are applied in the form of individual layers having a thickness on the order of magnitude of a few nanometers. In this case, this mixing together (=homogenization) takes place, in particular, at the beginning of a heat-treatment process which is carried out after the material application. However, it is also primarily the aim of such a heat-treatment process to form the ohmic contacts. It is then crucially advantageous if, during this heat-treatment process, all of the relevant material components of the material for the two contact regions are present directly at the interface with the semiconductor regions. This provides significant assistance to the formation of the ohmic contacts.
When the same material is applied to the first and second semiconductor regions, it may be possible, depending on the SiC doping chosen and depending on the material used for the two contact regions, for a slightly mutually deviating material composition to be established in a first boundary region, adjoining the first semiconductor region, of the first contact region and in a second boundary region, adjoining the second semiconductor region, of the second contact region. This slight deviation stems from different exchange processes between the applied material and the first or the second semiconductor region.
If the applied material contains, for example, a material component which acts as a donor or acceptor in silicon carbide, then this material component will migrate to a certain extent into the first or second semiconductor region, where it is bound, for example, as dopant at the corresponding lattice locations. This mixing-together process is critically influenced by the original doping of the first or second semiconductor region and thus proceeds differently in the first and second boundary regions. As a result, the proportion of the relevant material component at least in the first and second boundary regions changes to a mutually different, although very small, extent.
A displacement of the material composition likewise results in the boundary region of the first or second contact region if the material contains a siliciding material component. In this case, silicon originating from the first and second semiconductor regions is mixed together with the material of the first and second contact regions. As a consequence, a silicide based on the material component and the silicon is then formed, inter alia, in the two boundary regions. The degree of doping and the conduction type are critical influencing factors in this process as well, so that this effect can likewise lead to a slightly mutually deviating material composition in the first and second boundary regions.
The above-described interface effects are not manifested in those localized regions of the two contact regions which are remote from the interfaces. Therefore, the material is preserved in its originally applied composition and is thus the same in these regions of the first and second contact regions.
In the case of the teaching according to the invention, all slight differences in the material composition of the first and second contact regions, like those based on the above-described or similar interface effects, are not regarded as critical and are subsumed under the terms xe2x80x9cat least approximately identical material compositionxe2x80x9d and xe2x80x9cpractically homogeneousxe2x80x9d.
Moreover, differences in the material composition which are to be attributed to customary, optionally different, contaminants in starting substances are likewise regarded as non-critical in this case.
Furthermore, it is possible within the scope of the teaching according to the invention to apply slightly different material to the first and second semiconductor regions. However, as long as the material compositions deviate from one another by less than 10%, they are likewise designated as xe2x80x9cat least approximately identicalxe2x80x9d herein.
According to one advantageous embodiment, the applied material is composed of at least a first and second material component. In this case, the material may be present in the form of a mixture, a batch, an alloy or a compound of at least these two material components. The first material component advantageously is formed of a material which forms an ohmic contact on n-conducting silicon carbide with a contact resistance xe2x89xa610xe2x88x921 xcexa9cm2 and preferably xe2x89xa610xe2x88x923 xcexa9cm2. The second material component, on the other hand, contains at least one element of the third main group of the Periodic Table. The first material component, on one hand, produces a stable ohmic contact on that semiconductor region which is n-conducting. The second material component, on the other hand, produces a stable ohmic contact on the p-conducting semiconductor region.
In accordance with another feature of the invention, the second material component is present in a proportion by volume of from 0.1 to 50% in the material. A proportion of from 0.5 to 20% is preferred in this case.
As stated above, an embodiment in which the first material component at least contains nickel and the second material component at least contains aluminum is advantageous. An advantageous embodiment in which the material exclusively is formed of nickel and aluminum is distinguished, due to the nickel, by a good ohmic contact on the n-conducting semiconductor region. The admixture of aluminum with the nickel in the above-specified concentration range does not impair, or only slightly impairs, the contact resistance on the n-conducting semiconductor region. Moreover, the proportion of nickel in the material also prevents the formation of liquid aluminum islands and a resulting undesirable evaporation of aluminum during the heat-treatment process which is advantageously carried out for the purpose of forming the ohmic contact.
As an alternative to nickel, it is also possible, on one hand, to use one of the elements tantalum, titanium, tungsten, molybdenum, chromium, cobalt, iron or another transition metal and compounds of these elements as the first material component. The second material component, on the other hand, may also contain other elements of the third main group of the Periodic Table instead of aluminum, such as boron, gallium, indium or thallium. Other preferred materials for the contact-connection are thus composed of tantalum or tungsten as the first material component and of boron or gallium as the second material component.
Advantageous embodiments of the method, which emerge from the corresponding subclaims, have essentially the same advantages as the above-mentioned respectively corresponding developments of the semiconductor configuration itself.
Other embodiments of the method relate to the application of the material to the first and second semiconductor regions.
In accordance with another mode of the invention, the first and second contact regions are produced simultaneously, which is particularly advantageous. This considerably reduces the number of process steps required in comparison with successive application. Shorter production cycles can thus be achieved.
In accordance with a further mode of the invention, the material which is applied to the two semiconductor regions is taken from at least two separate sources. In this case, the sources each contain at least one material component, in particular the first or the second material component. They are taken by simultaneous vaporization or sputtering. The two contact regions are subsequently formed by depositing the material components on the first and second semiconductor regions. In this case, the material for the two contact regions is produced either while still in the vapor phase from the individual material components, in the course of the deposition process, or only thereafter. The process parameters can ensure adherence to a specific intended mixture ratio.
In accordance with an added mode of the invention, in contrast, a source material is firstly prepared from the first and second material components and then sputtered in a second method step. The released particles of the material form the two contact regions on the n-conducting and p-conducting silicon carbide, as in the previously described embodiment.
In accordance with an additional mode of the invention, the first and second material components are alternately applied in thin layers to the two semiconductor regions. This can be done by sputtering from two separate sources, so that alternately in a short time sequence, in each case only one of the two material components from the associated source is sputtered and deposited as a thin layer on the two semiconductor regions. The resulting thin layers have, in particular, only a thickness on the order of magnitude of a few xc3x85ngstrxc3x6ms. In the extreme case, such a thin layer may also be formed just of a single atomic layer, a so-called monolayer. Due to the small layer thickness and the short time sequence in the course of the layer deposition, this material application of the two material components is also designated as being simultaneous herein. Mixing together of the atoms of these monolayers (homogenization) then already takes place, depending on the process conditions, at least in part during the application process itself or right at the beginning of the subsequent heat-treatment process. Due to the small layer thicknesses, this mixing-together process only lasts a very short time.
In accordance with yet another mode of the invention, the semiconductor configuration is subjected to a brief heat-treatment process after the two contact regions have been applied. In this case, the semiconductor configuration is preferably heated to a maximum temperature of at least 500xc2x0 C., in particular of about 1000xc2x0 C., and then held at about this maximum temperature for up to 2 hours, in particular for 2 minutes. However, the heat-treatment process may also be formed only of a heating phase and an immediately following cooling phase, without a hold time at a maximum temperature being provided in between. This process serves for forming the two contact regions. It has been found that thermally stable contacts with good ohmic characteristics and a low contact resistance result both on the first and on the second semiconductor region, that is to say on n-conducting and p-conducting SiC, after this heat-treatment process.
In accordance with yet a further mode of the invention, the first and second contact regions are disposed on a common layer surface or on different layer surfaces. The above-described development possibilities and advantages of the contact-connection of n-conducting and p-conducting SiC with the same material apply analogously to both embodiments.
In accordance with a concomitant mode of the invention, the two contact regions are formed in such a way that they are contiguous or else separate. In this case, the two contact regions can be separated either as early as during the application of the material, by a corresponding mask technique, or afterwards by the removal of excessively applied material. Customary technologies such as etching, for example, may be considered for the latter process.
The two semiconductor regions that are to be contact-connected may be formed of different SiC polytypes. There are embodiments in which SiC in the form of 6H, 4H, 15R or 3C SiC is used for the two semiconductor regions. However, other polytypes are likewise possible.
Outside the first and second semiconductor regions, the semiconductor configuration may also be formed of a material other than SiC. Therefore, one embodiment provides at least one further semiconductor region, for example a substrate, made of a different material than SiC, for example made of silicon (Si), gallium arsenide (GaAs) or gallium nitride (GaN). This substrate is then integrated at least with the first and second semiconductor regions made of SiC to form a hybrid semiconductor configuration.
In one embodiment of the semiconductor configuration, the first and second contact regions are each situated at a freely accessible surface of the semiconductor configuration. This is not absolutely necessary, however. In other developments, the first and second contact regions may also be covered by layers applied in process steps that follow the contact-connection.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a semiconductor configuration with ohmic contact-connection and a method for contact-connecting a semiconductor configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.